Fluids that are contaminated with significant concentrations of acid-gases, such as, for example H2S or CO2, are herein referred to as sour. Conversely, fluids with substantially reduced or negligible concentrations of acid-gases are herein referred to as sweet. Sour fluids may occur naturally in both subterranean formations or at the surface, resulting from either thermogenic or microbiological processes. Sour fluids may also occur as a result of various industrial processes. For example, sour water may result from a process in which a sweet aqueous phase is used to strip acid-gases from a sour process vapor stream. Sour gas may result from a process in which sweet gas is used to strip acid-gases from a sour process water stream. Sour water may present many challenges and hazards related to its handling at the surface, including corrosion risks and human health and safety risks. For example, hydrogen sulfide contributes to corrosion of steel materials and can cause asphyxiation. However, it may be desirable that such sour water be reused for some subsequent process, for example, as a fluid to be used for hydraulic fracturing operations, so acid-gas removal may be desired.
There exist methods and systems available for treating sour water, particularly for removing hydrogen sulfide, but they are limited in application. Examples of chemical agents include scavengers such as triazines, ethanolamines, and acrolein, and oxidants such as chlorine dioxide and ozone. Generally, chemical scavenging agents either react with acid-gases irreversibly to form new compounds, or they react reversibly to form intermediate compounds through mechanisms of encapsulation, absorption, dissociation, or de-protonation. Chemical scavengers that follow some irreversible reaction mechanism are limited in application by a stoichiometric ratio of chemical required per molecule of target acid-gas present in the aqueous solution. As such, the amount of scavenger chemical required for sweetening increases proportionately with the amount of target acid-gas in solution. Such treatments can become uneconomic at exceedingly high concentrations of acid-gas, for example greater than 100 ppmw (“parts per million by weight”, commonly measured as mg/kg or mg/L in aqueous systems) of hydrogen sulfide in sour water. Other chemical scavengers that follow mechanisms of encapsulation, absorption, dissociation, or de-protonation tend to react reversibly such that a change in pH, pressure, temperature, or other stream conditions can lead to re-souring of the aqueous solution.
In some applications, gas stripping is a preferred method to remove volatile components, including dissolved acid-gases, from process liquid streams. Gas stripping is a process by which a stripping gas is introduced and mixed with a liquid process stream contaminated with volatile components, effecting a change in vapor-liquid equilibrium of the various system components relative to each other and resulting in a substantially reduced concentration of volatile components in the liquid process stream after separation of the mixed volatile components and stripping gas. Effectiveness of a gas stripping process is generally dependent on relative Henry's Law constants and may be improved by changing several process design factors such as increasing temperature, decreasing pressure, increasing the vapor-liquid contact efficiency, increasing the vapor to liquid molar flow ratio, and otherwise selecting a stripping gas composition and operating conditions so as to increase the mass transfer rate of acid-gas components from the process liquid into the injected stripping gas. Compared with chemical scavenger treatment mechanisms, gas stripping has the advantage of being more efficient at greater concentrations of acid-gas components and not being confined to stoichiometric limits. Being related to thermodynamic conditions, gas stripping treatment is designed to remove a fraction of target acid-gases that is independent of absolute acid-gas concentration. As such, the absolute removal of acid-gas components from a sour water stream increases as the initial concentration of acid-gas components in the stream increases, without requiring the addition of extra chemical or gas supply.
Efficient gas stripping may be obtained by using steam generated by boiling a portion of treated process water as the injected stripping gas. In other applications, air may be used as the stripping gas, and air may be pre-heated so as to increase the temperature of the overall system. Typically, these methods are used for applications in which gas stripping is used to remove low concentrations of acid-gases from otherwise substantially fresh water having a low concentration of total dissolved solid species that is, for example, less than 1,000 ppmw.
However, sour water resulting from oil and gas exploration and production operations commonly has substantial contamination from other components in addition to the souring components, such as hydrocarbons, suspended solids, salts, chlorides, multivalent cations, hardness, BTEX (commonly, the acronym referring to the combined species of benzenes, toluenes, ethylbenzenes, and xylenes), and the like. For oil and gas exploration and production operations in which hydrocarbons are likely to be present in produced water, maintaining an anoxic fluid system may be desired to reduce combustion risk associated with hydrocarbon and oxygen mixtures. Additionally, the total dissolved solids in water produced from hydrocarbon production wells, often in excess of 10,000 ppmw, and in some cases in excess of 100,000 ppmw, can result in scale formation on equipment internals. Boiling salt water with elevated total dissolved solids concentrations to generate steam to be used as a stripping gas is known to result in scale formation and solids deposition on equipment internals that decreases process efficiency and increases maintenance requirements. Thus, introducing an external stripping gas to a gas stripping process in replacement of steam may be desirable for treating waters with elevated total dissolved solids concentrations.
Efficiency of gas stripping is relatively independent of stripping gas composition, so long as the target acid-gas species has a similar affinity for each of the gases being compared for a gas stripping application. As such, natural gas, methane, air, steam, and nitrogen all represent viable options for stripping gas selection. Generally, the important requirement is that the stripping gas is substantially void of contamination by the target acid-gas component. Thus, the generally accepted requirement is that clean stripping gas is introduced to the process from an external source. However, in some applications, it may be desirable not to introduce an outside gas to the sour water treatment process, as this can add substantial cost and operational complexity that may make treatment uneconomic and inefficient.
Generally in oil and gas production operations, salt water is produced in the same mixed production stream with oil, natural gas, and natural gas condensate. Salt water is then separated from the mixed production stream to isolate hydrocarbon components of value. The existence of sour water from oil and gas extraction activities is the result of the mixed production stream also being sour. Because hydrogen sulfide and carbon dioxide gases partition into both the hydrocarbon and aqueous phases based on thermodynamic equilibrium conditions, the hydrocarbon split from a sour mixed production stream is generally also necessarily sour. As such, the natural gas stream that is produced with and is located in proximity to a sour produced water stream tends also to be sour. Such sour gas typically must be treated to remove acid-gas components before it can be used as an effective gas stripping agent because the presence of acid-gas components in the gas phase interferes with the stripping efficiency in a gas stripping process for sour water.
Completeness of gas stripping for removing acid-gases from sour water, although aided by the aforementioned process variables, is still limited by the equilibrium conditions imposed by Henry's Law and related vapor-liquid equilibrium considerations. Gas stripping targets and removes only volatile components dissolved in the process liquid and does not affect non-volatile components. Such non-volatile components may include the conjugate bases that form by the dissociation of acid-gases dissolved in water, for example the bisulfide and sulfide ionic derivatives of hydrogen sulfide, or the carbonic acid, bicarbonate, and carbonate species that are products of the reaction between carbon dioxide and water. In general, the dissolved form of an acid-gas tends to dominate at lesser pH values, and the conjugate base form tends to dominate at greater pH values. Regardless of pH, however, solubility and vapor-liquid equilibrium conditions dictate that an amount of a dissolved acid-gas in water exists as the conjugate base form. As a result, a gas stripping process alone cannot physically remove all acid-gas derivatives from water.
In some applications, in particular in the case of treating sour water from hydrocarbon producing wells for reuse, complete and irreversible removal of substantially all dissolved acid-gas forms is desirable and necessary to ensure safe handling of the fluids. Removal of the gaseous species without removal of the associated conjugate base species leaves the potential for sour gas release if the treated water is blended in an acidized process unit downstream. An example of such a downstream process is an “acid spearhead,” an oil and gas well treatment operation associated with hydraulic fracturing. In this process, water blended at a very low pH, approximately less than 2, is used as a pre-conditioning treatment to dissolve certain rock minerals in a subterranean formation in preparation for hydraulic fracturing. In an application for which sour water is to be treated and reused for hydraulic fracturing, the blending of water with an acid to result in a pH of less than about 2 could result in fluid recontamination and subsequent gaseous release of an acid-gas, if the treated water has conjugate base remaining (see FIGS. 2A and 2B).
In such applications, in which substantially complete removal of high concentrations of acid-gas components is desired, gas stripping pre-treatment may be combined with a subsequent aqueous-phase treatment process, such as the injection of acid-gas scavenging chemical, to ensure complete removal of acid-gases and associated conjugate bases.